1. Field of the Invention
Embodiments of the invention relate generally to the field of optical light guides and, more particularly, to non-imaging, light collecting and emitting apparatus, methods, and applications. Even more particularly, embodiments of the invention relate to a light collection and light guide apparatus for use in a concentrated photovoltaic (CPV) solar energy system, and other applications.
2. Related Art
Solar energy is an important part of the renewable energy solution. Concentrated photovoltaics (CPV) have the potential to provide a source of cost effective and clean energy. By concentrating solar energy with optics, less photovoltaic (PV) material is used, reducing cost, since PVs are expensive and energy-intensive to produce compared with optical components.
Co-owned U.S. Pat. No. 7,817,885 entitled LIGHT COLLECTION AND CONCENTRATION SYSTEM, the subject matter of which is incorporated by reference herein in its entirety, discloses a CPV system that incorporates a component light guide apparatus. The light guide apparatus includes a plurality of light directing structures 1102, 1104 (also referred to as ‘light injection elements,’ or air-prisms), shown by non-limiting, illustrative example in FIG. 1. FIG. 2 illustrates a planar light guide system as described in the related art in relation to a standard reference coordinate system. Incident light 1 from a distant, extended source (e.g., solar radiation) propagating generally in the (−)y (axial) direction is concentrated (e.g., light 2) by a lens 3 and injected into the light guide 4 via a light injection element 5 on or in the face of the guide. The light thereafter propagates generally in the z-direction (light-guiding direction) towards an exit end 6 of the light guide. The discrete light injection element 5 is a surface portion of the light guide apparatus that may be made by a partial transverse lateral cut extending from a region of the bottom surface 7 of the light guide. Depending upon the tilt angle of the light injection element, the index of refraction of the light guide, and the index of refraction of the external interface of the injection surface, radiation may be totally internally reflected from the face of the light injection element.
Alternatively or in addition, a similar light injection element 1102 can exist as a surface of the light guide apparatus made by a partial transverse lateral cut extending from a region of top surface portion 1021 (see FIG. 1). For light injection element 1102, radiation 1132 from a primary concentrator (not shown) optically coupled to light injection element 1102 is intercepted by the light injection element. Shaded area 1103 represents a reflective coating on surface 1102 that reflects the incident light 1132 into the structure for subsequent TIR propagation within the light guide apparatus (in the z-direction) towards and out the exit-end 1150. The exact angular orientations of the light injection elements will depend upon the nature of the reflection process (e.g., reflective (direct or TIR), refractive, diffractive), primary lens f/#, and the transport structure index of refraction n2. The notched region behind the light injection element 1104 may, for example, be filled with a lower index dielectric material to facilitate TIR into the light transport structure.
Typical dimensions of the light injection elements are 130 μm-140 μm for the tilted reflecting surface, a base dimension of about 130 μm, and a height dimension of about 140 μm. Depending upon the length (z-direction) and width (x-direction) of the transport structure, there will be many light injection elements (1102, 1104, both), which necessarily exist in the transport structure.
In CPV applications, a general object of the system is to collect as much solar radiation as possible and concentrate that radiation as much as possible for input to a PV cell located at an output end of the apparatus. The presence of the light injection elements results in a non-ideal light guide since light propagation through the transport structure is hindered by interactions with downstream light injection elements. Light loss can occur by absorption or scattering at a light injection element, out-coupling of light at a light injection element, or étendue dilution from interaction with a light injection element. Further system objectives include maximizing primary concentrator acceptance angle, maximizing injection concentration, maximizing light guide concentration, and minimizing component and system weights and thicknesses.
The design of the light guide apparatus in the '885 patent may be considered a homogeneous design; i.e., the medium of the light guide layer and the injection facet layer are the same and thus have matching refractive indices. Light propagating with the greatest angle relative to the guiding axis will interact with the air prisms or dimples (see below) most frequently, and thus couple out of the guide more rapidly. This limits the light concentration that can be achieved in a homogenous system.
The inventors have recognized the benefits and advantages of a light collecting apparatus for use in a CPV system that is more efficient, lower in cost, higher performing, and easier to manufacture than previous apparatus, and an apparatus that can collect and emit light for lighting applications. The inventors have recognized the further benefits and advantages of a light guide apparatus in which those rays propagating with a high angle can be redirected to travel down the guide at a lower angle relative to the guiding axis for more efficient collection, propagation, emission, and overall system operation. This further would allow light to be contained in the guide for a longer propagation distance and thus increases the concentration that these systems can achieve.